Implement fftshift for Frequencies

Fixes #56.

One caveat is that, since this implementation of `fftshift` returns a FloatRange for efficiency, `fftshift(rfftfreq(n))` doesn't actually do any shifting. This seems like a feature, since one probably doesn't want to shift the output of `rfftfreq` in a code path that shifts the output of `fftfreq`. However, it also means that `fftshift(rfftfreq(n)) != fftshift([rfftfreq(n)])`. Thoughts?